1. Field of the Invention
The present invention relates to a link file generating program, a color conversion program and a link file generating apparatus and, more specifically, to a link file generating program, a color conversion program and a link file generating apparatus that are used for a color matching system.
2. Description of the Related Art
It is a conventional practice to print, by a printer, image data acquired by reading a document by a scanner or image data displayed on a display. The image data that is processed by a scanner or a display has its colors represented by three elements, that is, red (R), green (G) and blue (B). This color system is referred to as the RGB color system. The image data processed by a printer has its color represented by four elements, that is cyan (C), magenta (M), yellow (Y) and black (K). This color system is referred to as the CMYK color system. As the color systems for the image data to be processed differ between the two devices, a color conversion process is performed, in which the image data is converted from one color system to another.
Color management system has been known as a system enabling execution of the color conversion process among a plurality of devices. In the color management system, XYZ color system or L*a*b*color system, for example, is defined as a color system independent of any of the devices (device-independent). Each device is provided with a corresponding profile, so that image data of the color system dependent on that device (device-dependent) is converted to a device-independent color system. The profile is, by way of example, data that relates a color (dependent color) represented by the RGB color system to a color (independent color) represented by the XYZ color system, in the form of a matrix or a table.
More detailed description will be given with reference to the figures. FIG. 16 is a schematic diagram of a conventional color management system. Referring to FIG. 16, in order to illustrate the method of color matching between an input device 1401 and an output device 1407, a flow of image data is shown. Here, the image data reproduced by input device 1401 such as a CRT or a scanner is RGB data represented by the RGB color system, and the image data reproduced by output device 1407 such as a printer is CMYK data represented by the CMYK color system. As can be seen from this figure, the RGB data goes through various conversion processes at a color conversion processing portion 1403, to be eventually converted to the CMYK data.
First, the RGB data at input device 1401 is input to color conversion processing portion 1403, and converted to data of a device-independent color system. As described above, the device-independent color system includes the L*a*b* color system and the XYZ color system. Here, it is assumed that the data is converted to data represented by the XYZ color system (XYZ data). The conversion process includes a conversion using a matrix and a conversion using an LUT (Look Up Table), as shown in FIG. 16.
Then, the converted XYZ data is converted to XYZ data in a range that can be reproduced by output device 1407, by a Gamut mapping portion 1405. Specifically, here at the Gamut mapping portion, color matching between input device 1401 and output device 1407 is performed.
The data after color matching is still the data of the device-independent XYZ color system (XYZ data), and therefore, the data is again converted to the CMYK data. Here again, the conversion process includes a conversion process using an LUT (Look Up Table) or a conversion process in accordance with masking method.
In this manner, the image data reproduced by input device 1401 is once converted to data of a device-independent color system, and thereafter, color matching is performed to enable reproduction by output device 1407.
There is a problem, however, that hue distortion results from the color conversion in the color management system. In order to cope with this problem, Japanese Patent Laying-Open No. 2002-152536 describes a technique in which, as the profile to be used for the conversion of image data in a device-dependent color system to data in a device-independent color system, a profile having hue distortion corrected in the device-independent color system is generated.
As already described, the profile relates device-dependent colors to device-independent colors. In order to reduce data amount, not every device-dependent color is related to the device-independent color. Therefore, when conversion to data in a device-independent color system is made, data that is not defined by the profile is calculated by interpolation using defined data. For the interpolation, cubic interpolation, for example, is used. The cubic interpolation will be described by way of example. Assume that in a profile of an input device, device-dependent RGB color system and device-independent L*a*b* color system are related to each other, and in a profile of an output device, device-independent L*a*b* color system and device-dependent CMYK color system are related to each other. Here, consider an example in which “red” in the RGB color system (RGB=(255,0,0)) is to be reproduced in the CMYK color system in which the corresponding “red” is given as CMYK=(0,255,255,0).
By a conversion process in accordance with a matrix or an LUT using the profile of the input device, RGB=(255,0,0) is converted, for example, to L*a*b*=(52.2,80.1,67.2). As L*a*b*=(52.2,80.1,67.2) is not defined in the profile of the output device, cubic interpolation is performed, using values of surrounding 8 points that are defined. The surrounding 8 points may be L*a*b*=(48,80,64), (64,80,64), (48,96,64), (64,96,64), (48,80,80), (64,80,80), (48,96, 80) and (64, 96, 80). These 8 points are slightly away from red, and therefore, the CMYK data in the CMYK color system obtained by the cubic interpolation using these points is not CMYK=(0,255,255,0). The cubic interpolation is a conventionally well-known technique, and therefore, detailed description thereof will not be repeated here.
As described above, there is always an error resulting from the interpolation, and hence image data after color conversion may have hue distortion due to the error. Particularly, when the image data includes a gradation area that has the same hue but brightness or chroma changed gradually, there is a problem that the gradation is hard to reproduce.
In the technique described in Japanese Patent Laying-Open No. 2002-152536, conversion is performed using a profile, and therefore, it is possible to prevent hue distortion for the data defined by the profile. Data that are not defined, however, are interpolated, and therefore, there is a possibility of hue distortion caused by the error resulting from the interpolation.